Process for developing key messages: The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops, teleconferences, and email exchanges. Authors considered inputs and comments submitted by the public, the National Academies of Sciences, and Federal agencies. For additional information on the overall report process, see Appendices 2 and 3.

Many water-related illnesses are of critical importance globally, such as cholera and hepatitis E virus, and they affect U.S. interests abroad, but the focus of this chapter is to address climate impacts on water-related illnesses of primary importance to human health within the United States. In addition, although climate change has the potential to impact national as well as global seafood supplies, this chapter does not cover these types of impacts because the peer-reviewed literature is not yet robust enough to make connections to human health outcomes in the United States. Even with those constraints, the impacts of climate on water-related illness are regionally or locally specific and may include increased risks as well as benefits. For example, the projected geographic range shifts of some Gambieridiscus species to more northern latitudes may mean that dominant ciguatera fish poisoning toxins enter the marine food web through different species, with increases of toxins in new areas where waters are warming and potential decreases in areas such as the Yucatan and eastern Caribbean Sea.1dfd14e0-eae8-46d9-9c3e-0fa3f0c37da4

New information and remaining uncertainties: Uncertainty remains regarding the relative importance of additional factors that may also act on naturally occurring pathogens and harmful algae at local or regional levels to influence their growth, distribution, and toxicity. In many cases, it is uncertain how these multiple factors may interact with each other to influence the seasonal windows and geographic range for pathogens and harmful algae, especially in dynamic coastal marine environments. For example, changes in salinity, competition with other plankton, and presence of viruses or other organisms that consume plankton or bacteria can affect abundance.d0d412e3-c46b-45a3-b4e5-c3e54c896c36cb3ad7db-a5a1-473e-bda9-ff18f5f06b6f Changing distribution patterns for some marine species of harmful algae is not well understood and some regions may become too warm for certain species of harmful algae to grow, shifting (without changing in total size) or even shrinking their geographic range.

Additionally, there are limited studies on projections for changes in illness rates due to naturally occurring waterborne pathogens and harmful algae. Uncertainty remains regarding appropriate methods for projecting changes in illness rates, including how to integrate considerations of human behavior into modeling (current methods to assess exposure risk assume similar human behavior across time scales and geography). Methodological challenges are related to 1) underreporting and underdiagnosis of cases that affect the accuracy of baseline estimates of illness, 2) ability to project changes in strain virulence, 3) accounting for the effects of potential adaptation strategies/public health interventions (for example, public service announcements on how to avoid exposure), and 4) accounting for changes in public healthcare infrastructure and access that can reduce the risk of exposure or illness/death if exposed.

Assessment of confidence based on evidence: Based on the evidence, there is medium confidence that, with changing climate, the annual seasonal and the geographic range for Vibrio and certain marine harmful algae will expand. The assessment of medium confidence is due to less certainty from modeling results regarding the magnitude of projected changes in abundance. The conclusions were deemed very likely to occur for Vibrio and likely for marine harmful algae based on good levels of agreement found in the published quantitative modeling projections for both Vibrio and marine harmful algae (Alexandrium and Gambieridiscus) cited above. This conclusion takes into consideration that for some marine algae (for example, Gambieridiscus), lower latitudes may become too warm and risk may decline in those areas as it increases at higher latitudes. For freshwater harmful algae, there is highconfidence that annual season and geographic range will expand with changing climate, which will also prolong the time for exposure and the potential for public health impacts. Consistent and high-quality evidence from a limited number of laboratory studies, modeling efforts, field surveys, and comparisons of historic and contemporary conditions support this assessment. The conclusion was deemed very likely to occur for freshwater harmful algae with high confidence based on laboratory studies and field observations, as well as a greater fundamental understanding of inland hydrodynamics and bloom ecology as indicated in the literature cited in the chapter. There is medium confidence regarding increased risk to human health from a longer potential time for exposure to waterborne pathogens and algal toxins and potential exposure for a wider (or novel) population. This confidence level was chosen due to less certainty stemming from a relative lack of quantitative data and projections for future illness rates in the peer-reviewed literature.